1. Field of the Invention
The present invention relates in general to an epoxy resin composition. More particularly, it relates to an epoxy resin composition containing a non-halogen, non-phosphorus flame retardant.
2. Description of the Related Arts
In recent years, environmental safety and in particular the air pollution has attracted worldwide attention. In this connection, electric and electronic appliances are required to be more environmentally friendly in addition to the requirement of high flame retardancy. To be more specific, the electric and electronic appliances are required to be resistant to flame and, at the same time, not to generate noxious gases. It has been customary in the past to use a glass/epoxy as a substrate of a printed circuit board on which electric and electronic appliances are to be mounted. In general, a brominated epoxy resin containing bromine as a flame retardant is used for forming the substrate of the printed circuit board.
The brominated epoxy resin certainly exhibits high flame retardancy. However, it generates noxious hydrogen bromide, polybromine dibenzofurans, and poybromine dibenzodioxins when burned. Moreover, antimony trioxide (Sb2O3), a synergist commonly used in company with brominated epoxy resin has recently been labeled as a suspected carcinogen.
To overcome this difficulty, epoxy resin compositions containing non-halogen flame retardants such as nitrogen compounds, phosphorus compounds have been developed. However, these flame retardants have the disadvantage that, in the instance of fire, they disintegrate releasing corrosive and partly toxic compounds like nitrogen oxide and derivatives of phosphoric acid. Thus, changing the brominated flame retardants into phosphorus or nitrogen-containing flame retardants does not achieve the goal of an environmentally friendly retardant.
The present invention is intended to provide an epoxy resin composition which does not contain a halogen, phosphorus, or nitrogen element but exhibits good flame retardancy.
Therefore, an object of the invention is to provide a halogen-free epoxy resin composition containing a non-halogen, non-phosphorus flame retardant, which does not release corrosive or toxic compounds in the instance of fire and at the same time has good flame retardancy.
The above and other objects are achieved by providing an epoxy resin composition comprising:
(a) 100 parts by weight of an epoxy resin;
(b) 40-60 parts by weight of a phenolic novolac hardener; and
(c) 5-60 parts by weight of a silica-novolac hybrid resin solution as a flame retardant.
In the present invention, the silica-novolac hybrid resin solution (c) is a reaction product obtained by a sol-gel reaction between (c1) an organosilane and (c2) a phenolic novolac resin.
The flame-retardant epoxy resin composition of the present invention is suitable for application in printed circuit board industry to make environmental-friendly prepregs. Besides, the epoxy resin composition can find applications in other electronic industry as packaging materials.
The present invention will be described in detail below.
The epoxy resin composition of the present invention contains a bisphenol A type epoxy resin as the component (a). As widely known to the art, the bisphenol A type epoxy resin is a reaction product between bisphenol A and, for example, epichlorohydrin. The bisphenol A type epoxy resin used in the present invention generally has an epoxy equivalent between 150 and 1000. The bisphenol A type epoxy resin used in the present invention is commercially available including, for example, EPIKOTE series manufactured by Yuka Sell Inc., Japan, and ARALDITE series manufactured by Ciba Geigy Inc. It is possible to use a single kind or a plurality of different kinds of the bisphenol A type epoxy resin in the resin composition of the present invention.
According to another aspect of the invention, the component (a) can be a mixture of a bisphenol A type epoxy resin and a novolac type epoxy resin. As widely known in the art, the novolac type epoxy resin is a resin obtained by a reaction between a novolac resin and epichlorohydrin. The novolac type epoxy resin used in the present invention should desirably have a softening point of 70-130xc2x0 C., more preferably, 80-100xc2x0 C. Such a resin is commercially available from, for example, Novolac epoxy series sold by Dow Company. It is possible to use a single kind or a plurality of different kinds of the novolac type epoxy resin in the resin composition of the present invention.
In the resin composition of the present invention, a phenolic novolac hardener is used as the component (b). The phenolic novolac resin is obtained by a condensation reaction between a phenolic compound and formaldehyde, which is carried out in the presence of an acidic catalyst. The phenolic compounds used for producing the novolac resin by the reaction with formaldehyde including, for example, phenol, cresol, and bisphenol A. The phenolic novolac resin used in the present invention has a molecular weight ranging from 100 to 30,000, more preferably, from 300 to 3,000. The phenolic novolac resin desirably has at least two phenolic hydroxyl groups in its molecule. It is possible to use a single kind or a plurality of different kinds of the phenolic novolac resin in the resin composition of the present invention.
The resin composition of the present invention contains a silica-novolac hybrid resin solution as a flame retardant (c). The silica-novolac hybrid resin solution is a reaction product obtained by a sol-gel reaction between (c1) an organosilane and (c2) a phenolic novolac resin. To be more specific, the silica-novolac hybrid resin is obtained by reacting a phenolic novolac resin with a nano-scale silica cluster derived from the organosilane, so that the phenolic novolac bonds to hydroxyl groups on the silica cluster to form a silica-based, multi-functional novolac resin.
The organosilanes (c1) used for producing the flame retardant of the present invention have the following general formula:
R1nSi(OR2)4xe2x88x92n
wherein n is 0, 1, 2, or 3; R1 is alkyl having a terminal functional group selected from epoxy, alkenyl, amino, carboxy, or hydroxy; and R2 is alkyl. Illustrative of organosilanes suitable for use herein are tetraethyl orthosilicate (TEOS), 3-aminopropyltriethoxy silane, and glycidyloxypropyltrimethyl silane.
The phenolic novolac resin (c2) is obtained by a condensation reaction between a phenolic compound and formaldehyde, which is carried out in the presence of an acidic catalyst. The phenolic compounds used for producing the novolac resin by the reaction with formaldehyde including, for example, phenol, cresol, and bisphenol A. The phenolic novolac resin used in the present invention has a molecular weight ranging from 100 to 30,000, more preferably, from 300 to 3,000. The phenolic novolac resin desirably has at least two phenolic hydroxyl groups in its molecule.
In carrying out the sol-gel reaction, the molar ratio of organosilane (c1) to phenolic novolac (c2) is preferably from 2 to 20, more preferably from 4 to 16. The reaction is suitably carried out in the presence of an acidic or basic catalyst such as hydrochloric acid, sulfuric acid, acetic acid, or ammonium hydroxide. The amount of the catalyst used is preferably in a range of 0.1-2 parts by weight, based on 100 parts by weight of the epoxy resin (a). The reaction is suitably carried out at a temperature ranging from 30 to 90xc2x0 C., more preferably from 60 to 80xc2x0 C.
The flame retardant of the present invention can form a supporting structure to support chars generated by phenolic novolac resins when burned, thereby separating inflammable components (e.g., epoxy resins) from heat.
The epoxy resin composition of the present invention may further comprise a curing accelerator (d) commonly used for accelerating the curing of an epoxy resin. To be more specific, the curing accelerator (d) includes, for example, imidazole compounds such as 2-ehtyl-4-methylimidazole and 1-benzyl-2-methylimidazole; and tertiary amines such as Nxe2x80x2,N-dimethylbenzylamine (BDMA). These compounds can be used singly or in the form of a mixture. The curing accelerator should be used in a small amount as far as the accelerator is sufficient for accelerating the curing of the epoxy resin. The amount of the curing accelerator used is preferably between 0.1 and 1 parts by weight based on 100 parts by weight of the epoxy resin (a).
A prepreg can be manufactured from the epoxy resin composition of the invention by the ordinary method. Specifically, the resin composition is diluted with a suitable organic solvent to prepare varnish, followed by coating or impregnating a porous glass substrate such as a glass nonwoven fabric or a glass woven fabric with the varnish and subsequently heating the substrate to obtain a desired prepreg. Examples of suitable organic solvents for the dilution include N,N-dimethylformamide, acetone, isopropanol, ethylene glycol monomethyl ether, propylene glycol monomethyl ether, butanol, and methyl ethyl ketone. The prepreg thus obtained can be used for manufacturing a cooper-clad laminate, a multi-layered laminate, and a printed circuit board by conventional methods well known in the art.